Tip #1: Look at the Color Wheel
Complimentary colors are a great two colors to work with when color blocking. For example, use purple and yellow as your two colors and pair a purple top with a cute yellow jacket. The picture below does exactly this, in order to style a lovely outfit.
Tip #2: Bring out each color by using the same color somewhere else in the outfit.
In the same photo, we can easily see that the purple shoes and shirt compliment each other beautifully, as does the yellow in the shorts and the yellow jacket.